Microwave frequency selective apparatus



United States Patent Ofiiice 3,027,525 Patented Mar. 27, 19%2 3,927,525 MICROWAVE FREQUENCY SELECTIVE APPARATUS Edward Salzberg, Framingham, Mass, assignor to Microwave Development Laboratories, Inc., Wellesley, Mass,

a corporation of Massachusetts Filed Apr. 28, 1958, Ser. No. 731,207 12 Claims. (Cl. 333-73) The present invention relates in general to frequency selective apparatus for transmitting microwave signals and more particularly concerns a compact waveguide structure arranged to provide exceptionally good band pass and rejection characteristics. Three zeroes of the reflection coefficient are introduced by a waveguide section only a quarter wavelength long.

In a representative prior waveguide filter, three zeroes of the reflection coefiicient are introduced by spacing three resonant windows a quarter wavelength apart in a waveguide section a half wavelength long. The resonant frequencies of the respective windows occur at the cener frequency of the filter. For applications where space and weight are at a premium, such as in airborne radar systems, the relatively large volume required for such filters is disadvantageous.

The present invention contemplates and has as a primary object the provision of a more compact microwave filter of increased selectivity characterized by three reflection coefiicient zeroes within the filter bandwidth.

Another object of the invention is to provide a filter in accordance with the preceding object having exceptionally low insertion loss in the filter pass band.

Still another object of the invention is to provide a microwave T-R tube providing a high degree of imedance match over a very wide bandwidth.

A further object of the invention is to provide a microwave switch having the features described in the preceding object.

According to the invention, the filter comprises a waveguide section having a resonant element midway between the ends of the section and like reactive elements symmetrically disposed on opposite sides of the resonant element.

In one form of the invention, the section is only a quarter wavelength long with capacitive irises at opposite ends thereof and a resonant window situated at the midpoint of the section.

Other features, objects and advantages will become apparent from the following specification when read in con nection with the accompanying drawing in which:

FIG. 1 is a perspective view of a waveguide incorporating the principles of the invention with portions cut away to expose the reactive elements included in the novel filter;

FIG. 2 is a view along section 2-2 of FIG. 1;

FIG. 3 is a graphical representation of reflection co efficients as a function of frequency;

FIG. 4 is a sectional view of a waveguide section having cascaded filter sections according to the invention;

FIG. 5 is a sectional view of a resonant window especially suitable for use in a T-R tube or switch incorporating the inventive concepts; and

FIG. 6 shows the band pass characteristic of the filter of FIG. 1 compared with that of a prior filter having three resonant windows spaced by a quarter wavelength.

Where applicable, like elements in the dilferent figures are identified by the same reference numeral.

With reference to the drawing and more particularly FIG. 1 thereof, a waveguide 11 is shown with portions cut away to expose the reactive elements comprising the novel filter within a quarter wavelength section thereof.

These elements include a resonant element 12 formed of conducting pltae 13 in edge contact with the top and bottom wide walls 14 and 15 and left and right narrow walls 16 and 17 and having a rectangular window 18. A pair of irises are formed in spaced planes 21 and 22 separated by a quarter wavelength and symmetrically disposed on opposite sides of resonant element 12 by conducting rectangular plates 23 and 24, respectively, both in edge contact with left and right narrow walls 16 and 17. A portion of plate 23 is cut away to expose window 18. Plate 24 is partially visible through window 18.

The arrangement of the reactive elements within the waveguide is further clarified in FIG. 2 which is a section view through the plane indicated by line 2-2 as viewed along the indicated direction.

As noted above, this arrangement provides three reflection coefficient zeroes within the filter pass band although the filter elements are located within only a quarter wavelength section of waveguide. In addition to conserving space, the novel filter exhibits an exceptionally high degree of selectivity and a low V SWR over the pass band.

With reference to FIG. 3, there is shown a graphical representation of reflection coefficients as a function of frequency in the plane of resonant window 12 to facilitate an explanation of the mode of operation of wide band filters arranged according to the invention. In FIG. 3A the solid line II represents the magnitude of the refiection coefficient due to resonant window 12. This window exhibits a resonance at a frequency f where II Q and the window admittance are zero. Above and below this frequency, the window admittance is capacitive and inductive, respectively, and non-zero.

At the frequency where planes 21 and 22 are spaced by exactly a quarter wavelength, the capacitive elements referred to the plane of window 12 exhibit a resonance where admittance and the magnitude of the reflection coeflicient due to these elements is zero. This frequency is shown in FIG. 3A as occurring at h, the resonant frequency of window 12. Above and below this frequency, the combined admittance of the capacitive elements re ferred to the plane of window 12 is inductive and capacitive respectively. Hence, the phases of I and P are opposite while their magnitudes are nearly equal from a frequency just below f to a frequency just above f Since the resultant reflection coefiicient P is the algebraic sum of I and P [F I is the amplitude difierence between ]I and 1I |P is graphically represented in FIG. 3B. Note that a zero occurs at f f and f where the II I and [P are equal. The particular frequencies where the zeroes occur may be controlled by appropriately adjusting the values of the various elements. Zeroes of IF I and ll l need not occur at f as shown. However, in the region between adjacent lowest frequency zeroes, T and P will be in phase and [F I will be the sum of II I and II I.

With reference to FIG. 4, there is illustrated a sectional view of a rectangular waveguide section for providing selective transmission characteristics according to the invention. Basically, adjacent resonant windows are spaced by a quarter wavelength midway between capacitive elements, adjacent capacitive elements also being spaced by a quarter wavelength. In FIG. 4, three resonant elements 41, 42 and 43 are shown midway between adjacent ones of capacitive irises 44, 45, 46 and 4-7. It is to be understood that any number of similarly cascaded sections may be formed in accordance with the invention.

Increasing the number of sections increases the number of available reflection coefficient zeroes. By appropriately choosing the dimensions :of the respective elements,

various types of filters may be provided having Butter- -shown, energy may pass throughthe filter.

worth maximally flat frequency response characteristics or Tchebyshev characteristics wherein ripples occur within and/or outside of the filter pass band.

Referring to FIG. 5, there is illustrated a sectional view of an alternative way of forming a resonant window as a substitute for resonant element 12 (FIG. 1) so that when a gas is sealed in the filter section including the capacitive elements (not shown in FIG. and this window, a T-R tube is provided capable of providing an excellent impedance match over an exceptionally wide band of frequencies. This resonant window comprises a pair of inductive conducting posts 61 and 62 in contact with upper and lower wide walls 11.4 and and a pair of closely spaced pointed capacitive probes 63 and 64 in conductive contact with upper and lower walls 14 and 15, respectively. When the transmitter is on, the breakdown potential across the gap between the points is exceeded and conduction occurs to prevent transmitted power from reaching the receiver.

This arrangement may also be adapted for use as a microwave switch by arranging probes 63 and 64 relatively movable. When the two points are spaced as When the two are in contact as indicated by dotted lines 65, the flow of energy through the waveguide section is almost completely impeded by the resultant short circuit. A solenoid or other suitable m ans may be utilized to control the relative position of probes 63 and 64.

With reference to FIG. 6, there is shown the frequency response characteristic 51 of a bandpass filter of the type shown in FIG. 1 having three reflection coefficient zeroes in a pass band three kilomegacycles Wide at X band. At frequencies within the pass band, the VSWR is less than 1.1. This may be compared with the response 52 of a conventional filter having three resonant windows spaced by a quarter wavelength. Note, that the rate of attenuation provided by both filters is nearly the same but the embodiment of the invention provides a much wider pass band and has. a lower VSWR, yet occupies half the volume.

Since those skilled in the art may make numerous modifications of and departures from the specific embodiments described herein without departing from. the inventive concepts, the invention is to. be construed as limited only by the spirit and scope of the appended claims.

What is claimed is;

1. Microwave transmission apparatus for selectively filtering a limited range ofmicrowave signals centered about a predetermined mid-band'frequency comprising, a waveguide, a pair of capacitive elements disposed within said waveguide and spaced apart no more than a quarter wavelength axially of saidwaveguide, a resonant element within said waveguide disposed substantially midway between said capacitive elements, the resonant frequency of said resonant element being substantially offsetfrom said mid-band frequency.

2. Microwave transmission apparatus in accordance with claim 1 wherein said resonant frequency of said resonant element is substantially below said mid-band frequency.

3; Microwave frequency selective apparatus in accordance with claim 1 wherein said resonant element includes a pair of closely-spaced probes whereby the breakdown potential across the gap between said probes is exceeded when the microwave power within said frequency selective apparatus exceeds apredetermined value.

4. Microwave frequency selective apparatus in accordance with claim 3 wherein said probes are relatively movableto a mutually contacting positioneffectively'short circuiting said waveguide.

5. Microwave frequency selective apparatus comprising, a waveguide, a resonant element in said waveguide, and a pair of capacitive elements insaid waveguide symmetrically disposed on opposite sides of said window and coacting therewith to introduce three reflection coefiicient zeroes at different microwave frequencies, said resonant element and said capacitive elements being located 'within a section of said waveguide having a length within a quarter wavelength of energy vibrating in the guide at the resonant frequency of said resonant element.

6. Microwave frequency selective apparatus comprising, a waveguide, a resonant element in said wave-: guide resonant at a frequency within the pass band of said frequency selective apparatus, and a pair of capacitive elements in said waveguide symmetrically disposed on opposite sides of said resonant element and coacting therewith and with each other to introduce three reflection coefiicient zeroes within said pass band, said resonant element and said capacitive elements being located within a quarter wavelength section of said waveguide, said quarter wavelength referring to the length of wave energy in said guide vibrating at the resonant frequency of said resonant window.

7. Microwave frequency selective apparatus comprising, a section of waveguide, a pair of capacitive elements at opposite ends of said section, and a resonant element in said section midway between said ends, said section having a length within a quarter wavelength .of energy in said guide vibrating at the resonant frequency of said resonant element, the combined reflection coefficients of said capacitive elements referred to the plane of said resonant element being of substantially equal magnitude and opposite phase with respect to the reflection coefficients of said resonant element in said plane for at least two different microwave frequencies. I

8. For transmitting microwave energy within a selected frequency band, apparatus comprising, a waveguide section, a pair of capacitive elements at opposite ends of said section, and a resonant element in said section midway between said ends, said resonant element having a resonance at a predetermined frequency within said selected frequency band, the. spacing between, said capacitive elements. being substantially a quarter wavelength at said predetermined frequency, the effective impedance of said capacitive elements coupled to the plane including said resonant element by said waveguide, section having a resonance at said predetermined frequency.

9. Microwave frequency selective apparatus cornprising, a waveguide section dimensioned to normally propagate, microwave energy within a predetermined frequency spectrum, a resonant element in a transverse piano of said section having a reflection coefficient. of substan-' tially zero at a first frequency within said spectrum where said element resonates, and first and second reactive elements within said section equidistant from said transverse plane and separated by substantially a, quarter wavelength at a frequency within said spectrum, the combined reflection coefficients of said first and second reactive elements referred" to said transverse plane further combined with the reflection coefiicient of said resonant element in said transverse plane producing a reflection coefficient of substantially zero at second and third frequencies within. said spectrum.

10. Microwave frequency selective apparatus in accordance. with claim 9 wherein. the magnitude of said combined reflection coefficients of said first and second reactive elements is of approximately the same magnitude as said resonant element reflection coefficient but of opppsite phase within a contiguous portion of said spectrum including said first, second and third frequencies.

11. Microwave frequency selective apparatus in accordance with claim 10 wherein the combined reflection coefficients of said first and second reactive elements is substantially Zero at said first frequency.

12. Microwave frequency selective apparatus for selectively filtering a limited range of microwave signals centering about a predetermined mid-band frequency comprising, awaveguide section, a plurality of like reactive elements within said section, each of said. reactive e ements being non-resonant at frequencies within said limited range of microwave signals, the spacing between adjacent ones of said reactive elements being a quarter wavelength axially of said waveguide section, and a plurality ;of resonant elements, respective ones of said resonant elements being located midway between adjacent ones of said like reactive elements, the combined reflection coefficients of a pair of adjacent like reactive elements referred to the plane midway therebetween including one of said resonant elements being of substantially equal magnitude and opposite phase with respect to the reflection coelficients of said one resonant element in said plane for at least three different microwave frequencies.

References Cited in the file of this patent UNITED STATES PATENTS 2,413,963 Fiske Jan. 7, 1947 6 2,432,093 Fox Dec. 9, 1947 2,473,834 Tuller June 21, 1949 2,496,865 Fiske Feb. 7, 1950 2,706,275 Clark Apr. 12, 1955 2,749,523 Dishal June 5, 1956 2,819,391 Reiches Jan. 7, 1958 2,845,577 Smullin July 29, 1958 OTHER REFERENCES Ragan: Microwave Transmission Circuits, vol. 9, M.I.T. Radiation Laboratory Series, McGraw-Hill Book Co., New York. Copyright May 21, 1948 (pp. 677-706 relied on). 

